1. Field of the Invention
The subject invention relates to heat exchangers, and more specifically to an evaporator, that utilizes flat tubes having a plurality of flow passages extending therethrough.
2. Description of the Related Art
Evaporators for automobile heating, ventilation and air conditioning (HVAC) systems are well known in the art as described in the U.S. Pat. Nos. 4,470,455 and 4,535,839. Such evaporators typically include a core formed by a plurality of tubes between which fins are disposed for permitting ambient air to flow across the exterior of the tubes. The tubes are in fluid communication with spaced tanks to allow refrigerant—working fluid of the system capable of undergoing transformation from liquid to vapor and vice versa—to flow from one tank to the other through the tubes. This permits heat exchange between the refrigerant and the ambient air as the refrigerant flows through the tubes.
Various evaporator tubes exist in the art. For example, a laminated tube is fabricated by joining a pair of embossed plates together to create interior sidewalls that define a channel through which the refrigerant flows. The hydraulic diameter of such a channel is typically determined by multiplying the cross sectional area of the channel by four and dividing that result by the wetted perimeter of the channel. The relatively small hydraulic diameter of the channel and the embossed surfaces of the conjoined plates produce a relatively high convective heat transfer coefficient for the refrigerant flowing through the tube. Despite this advantage, laminated tubes have certain drawbacks. For example, the embossed patterns on the surfaces of the plates make it difficult for the fins to bond to the surfaces. Furthermore, the plates are expensive to fabricate and result in tubes that can be subjected to relatively low refrigerant side pressure.
Certain flat tubes with a plurality of non-circular flow passages fabricated by using extrusion techniques do exist, which are designed to address the drawbacks associated with the laminated tube evaporator as described in the U.S patents bearing the U.S. Pat. Nos. 5,318,114; 6,161,616 and 6,449,979. However, none of these patents deal with the optimal dimensions of the circular or noncircular refrigerant flow passages within the extruded flat tubes nor do they deal with the optimal number of tubes in each pass of a multi-pass evaporator. The present invention is directed at high performance flat tube evaporators with enhanced refrigerant side passages of optimal dimensions and optimal number of tubes in each pass of a multi-pass evaporator.
The dominant heat transfer mechanism within the prior art evaporators is forced convection boiling, which is driven by the flow of the refrigerant through the flow channels. Forced convection boiling typically includes four stages. The first stage, or bubbly flow regime, is that in which the vapor mass fraction of the refrigerant is very low. In the second stage, or slug flow regime, the vapor volume fraction increases and individual bubbles begin to agglomerate to form plugs, or slugs, of vapor that move through the tube. The third stage, or annular flow regime, occurs when the interior walls of the tube are covered with a thin film of liquid refrigerant through which heat is absorbed. The mist flow regime is the final stage. During this stage, there is a sharp reduction in the boiling heat transfer coefficient of the refrigerant within the tube. Throughout all four stages, a nucleate boiling regime exists in selected areas of the tube, which results in quasi pool boiling of the refrigerant in those areas. However, the prior art tubes are not designed to ensure that such boiling optimizes the amount of heat transferred through the tube.